Convert one more NULL to nullptr

https://google.github.io/styleguide/cppguide.html#0_and_nullptr/NULL
says:
"For pointers (address values), use nullptr, as this provides
type-safety."
Furthermore, NULL won't compile if we change |SomeClass*| to
|CheckedPtr<SomeClass>|. See go/miracleptr for project details.

Bug: 1080832
Change-Id: I75a25ed0ff64561cb68555f5a6622cbb5b9d723a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228741
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774570}

media/base/fake_audio_renderer_sink.cc

@@ -47,7 +47,7 @@ void FakeAudioRendererSink::Start() {
 }
 
 void FakeAudioRendererSink::Stop() {
-  callback_ = NULL;
+  callback_ = nullptr;
   ChangeState(kStopped);
 }